1. Field of the Invention
The present invention relates to a power unit of a shaft drive type vehicle. More particularly, the present invention relates to a power unit for shaft drive type vehicle in which an extended shaft, which is provided on a shaft end portion of an output shaft of the power unit, has a cam damper mechanism and a pair of bevel gears connecting a drive shaft system.
2. Description of the Background Art
There is a known power unit of a shaft drive type motorcycle, in which a countershaft of a transmission constitutes an output shaft, an extended shaft is coaxially provided on a shaft end portion of the output shaft protruding to the outer side from a crankcase, and the extended shaft has a cam damper mechanism and a pair of bevel gears connecting a drive shaft system. An example of such power unit is disclosed in the Japanese Patent Publication No. 4795182 (specifically in FIG. 2 thereof).
In the power unit structure, as shown in the Japanese Patent Publication No. 4795182, an outer end portion of the extended shaft is rotatably supported on a bevel gear via a bearing, and one of bevel gears, and the cam damper mechanism are mounted in series on the extended shaft.
When attaching the extended shaft, first, a cam face member on one side which constitutes the cam damper mechanism is fastened to a countershaft. Then, the outer end portion of the extended shaft is pierced and fastened to the bearing set on the bevel gear cover so as to sandwich one of the bevel gears. The bevel gear cover is fastened to the crankcase so as to be abutted thereon in such a state that a cam member on the other side, which constitutes the cam damper mechanism, and a damper spring are mounted.
Consequently, the inside end portion of the extended shaft is coaxially and reciprocally rotatably connected to the output shaft via a needle bearing. The cam member is abutted on the cam face member (lifter member), which is fitted on the countershaft (output shaft) side, and the damper spring is in a specified preloading compressed state.
Therefore, according to the power unit structure as shown in the Japanese Patent Publication No. 4795182, a cam damper can be provided without increasing the number of axis of the power unit, which is advantageous in the reduction in cost and weight. However, the damper spring is preloaded after the bevel gear cover is attached to the crankcase, and it is necessary to provide a shim between a bevel gear case and the bearing, or between the bearing and the bevel gear, one of the bevel gears, in order to put a pair of bevel gears in a specified meshing state. As a result, since both the shim and the damper spring are fastened and compressed together, there is a difficulty in confirming the preload of the damper spring and is a problem of the change in preload according to the thickness of shim.
In consideration of the above-described circumstances, it is one of the object of the present invention to provide a power unit of shaft drive type vehicle capable both of keeping the uniform preloading of a damper spring and facilitating the adjustment of the mesh amount of bevel gears, in a power unit which has a cam damper mechanism and a pair of bevel gears connecting the drive shaft system on the extended shaft provided on a shaft end portion of the output shaft of the power unit.